The Clockwork Rocket

subsequently bent by the comb.

Yalda worked with scrupulous care, but after a while the process became mechanical, automatic. She glanced at the polarisers she’d taken off the bench: slabs of an exotic form of clearstone from Shattered Hill. Place one of them in a beam of light, and the beam’s brightness was diminished by one third. A second polariser aligned identically with the first had no effect, but if the two were “crossed”—their axes arranged at right angles to each other—the original brightness was diminished by a further third.

Giorgio had sought to explain this in terms of the wave doctrine. An elastic solid could experience shear waves, in which the medium suffered distortions perpendicular to the direction of the wave’s motion. A polariser, he argued, must somehow be inhibiting light’s equivalent of such waves when they lined up with the stone’s special axis. A horizontal polariser could rid a beam of sunlight of its left-to-right vibrations; a second, aligned vertically, would rid it of all waves that vibrated up and down.

A mystery remained, though. Along with shear waves, every solid carried pressure waves, which were much like the sound waves in air. The velocities of the two kinds of wave were due to distinct properties of the material, and pressure waves always traveled faster than shear waves. It would require both a truly bizarre material and an absurd coincidence to force the two to share the same speed.

When two crossed polarisers were held up to a star trail, if the light that emerged had traveled from the star at a different speed than the light that was blocked, some portion of the trail’s spread of velocities should have been favored over the rest. But in fact what was seen was a perfectly uniform dimming of the entire trail. Light waves that lacked polarity—supposedly the equivalent of a solid’s pressure waves—were no faster or slower than the rest.

Yalda could not believe that this was a coincidence—a perfect conspiracy of elastic moduli. Rather, what it suggested was that the whole analogy was flawed. Whatever carried light between the stars wasn’t actually being squeezed and stretched and sheared. Nereo had pinned down light’s wavelength, the distance at which each cycle repeated, but the truth was that no one yet had an answer to the question: cycles of what?

When Yalda had a full range of measurements, she sprinkled dye onto her chest and made three copies of the figures on paper: one for Giorgio, one for Nereo—not much use to him, since the numbers were tied to a particular slab of clearstone, but an appropriate gesture nonetheless—and one to keep in the workshop alongside the prism.

Nereo was waiting at the university’s southern gate, an ornate stone archway encircled by violet-flowered vines that had been bred to open their blossoms even in daylight. Yalda thanked him profusely, and almost offered to carry his luggage to the station, but Rufino and Zosimo were already grappling with the cases, and she’d learned not to wound their pride with gratuitous displays of physical prowess.

When they’d left, Giorgio upbraided her sternly before finally conceding, “I suppose it was worth it in the end. You’re not much of a diplomat, but this could yield interesting results.”

The understatement was insulting, but Yalda didn’t push her luck. “I hope so,” she said.

Giorgio regarded her with weary affection. “And I hope you’re ready to try displaying a bit more tact.”

“Of course!” Yalda said, genuinely chastened now. “The next visitor, I promise—”

Giorgio hummed, annoyed. “Forget about the next visitor! You want to use the telescope, don’t you?”

“Yes.” Yalda was bewildered; did he mean that she’d probably be up on the mountain, unable to cause any more embarrassment when his next guest came to speak?

Then she understood.

“The next unallocated slot at the observatory begins in seven stints,” Giorgio said. “If you want that slot for your wavelength measurements, you know who you’re going to have to deal with.”

A motif of two entwined helices was carved into the wall outside Ludovico’s office. The curves represented the motion of Gemma and Gemmo, co-planets that circled a common center every eleven days, five bells, nine chimes and seven lapses. Of course, they also traveled around the sun, and their distance from the world rose and fell substantially during each six-year orbit. Before Yalda had even been born, Ludovico had noticed that as Gemma and Gemmo drew farther away, the precise clockwork of their mutual circling seemed to slow, very slightly: the observed times when one planet crossed in front of the other slipped behind the predictions of celestial mechanics. But Ludovico had realized that the laws of gravity were not at fault; the light was just taking a little longer to arrive. With this insight, his observations had allowed him to compute the first reliable figures for the speed of light, averaged across the colors.

By the time Ludovico called Yalda into his office, the sun had set. He’d lit a firestone lamp, which sat sputtering and sizzling on a corner of his grand, paper-strewn desk. Standing before him, eyes respectfully downcast, Yalda summarised her proposal quickly. Her aim, she declared, was to correlate the angles of separation in a star trail with the angles of deflection produced by a clearstone prism; there was no need to mention Nereo’s device at all. “If I can find the formula that links the prism’s effect with the light’s velocity, that might lead to some insight into the mechanism of chromatic deflection.” In fact the data she gathered would be perfectly suited to that purpose; she was not really being dishonest.

When she’d finished speaking, Ludovico emitted a muted hum, the tone signifying gratitude that a tedious ordeal had finally ended.

“I’ve never had much time for you, Yalda,” he said. “Not because you hail from the benighted eastern provinces, with your quaint dialect and bizarre customs; that can be endearing, and even correctible. And not because you’re a woman—or almost a woman, or something that might have been a woman if nature had taken its proper course.”

Yalda looked up, startled. She hadn’t been insulted in quite such an infantile fashion since she’d left the village school.

“No, what I find objectionable is your arrogance and your utter inconstancy. You hear of an experiment, you read of some research, and whatever ideas you’ve supported in the past fly out the window. You simply trust in your own infallible powers of reasoning to guide you to the truth, as you swerve this way and that.” Ludovico held up a hand and made a zigzagging motion. “Well, I’ve heard of all the same experiments, I’ve read all the same research. I suppose I must not share your hubris, though—because I’m not driven to the same undignified series of self-contradictory declarations and endless changes of allegiance.”

Yalda said nothing, but she struggled to recall what she might have done to earn this tirade. At her admission interview, where Ludovico had sat on the panel, she’d professed some sympathy for the particle doctrine; that had been before Giorgio’s double-slit experiment. But at a debate half a year ago, she’d taken the side of the wave doctrine, and expressed the flaws in the opposing view quite forcefully. Why not? The evidence had mounted up, and she’d found it increasingly compelling. But apparently it was a kind of arrogance, to trust her feeble powers of reasoning to bring her to that conclusion.

Ludovico reached down to a shelf below his desk and lifted up a bulky stack of paper. In fact it was a book, Yalda realized, though the binding was in a terrible state.

“Have you ever read Meconio on the theory of luminous corpuscles?” he demanded.

“No sir,” Yalda admitted. Meconio had been a philosopher in the ninth age; she’d heard that he’d made some minor contributions to the study of rhetoric, but his grasp of natural phenomena had been less than impressive.

“If you can produce a halfway perceptive, three-dozen-page essay on Meconio within the next two stints, I’ll allow you to use the observatory.” Ludovico held out the tattered book; Yalda reached across and took it carefully. “A little exposure to a truly great mind might finally endow you with a trace of humility.”

“Thank you, sir. I’ll do my best.”

Ludovico hummed irritably. “If you can’t manage a commentary that’s worthy of my attention, leave the book with my assistant and don’t ever waste my time again.”

Yalda left his office and trudged down the dark hallway toward the exit. Two bells ago, she’d been euphoric; now she just felt hopeless. This man had set her an impossible task; even if Meconio’s tome was strewn with dazzling insights worth praising to the sky, she would never be able to plow through so much fusty ninth-age